The invention relates generally to circuits for converting a.c. to d.c. voltage and particularly to a converter which utilizes phase controlled demodulation.
In the field of rectification, it is well known to rectify an a.c. voltage by means of a rectifier circuit composed of diodes. While there are many such diode rectifier circuits, one such circuit which has proved to be quite useful is the so called two way rectifier bridge. Such circuits, however, suffer from the disadvantage that any d.c. component in the input also appears in the output. In addition, these circuits typically use large size capacitors in order to reduce a.c. ripple in the output. Accordingly, when the circuit is first turned on, a time period is required in order to charge the capacitor before full output voltage is achieved. As such, this type of circuit is not suitable for use in instrumentation where the input signal may change rather rapidly with respect to the speed at which the filter capacitors can be either charged or discharged.
One approach to partically overcoming the problem of such bridge rectifier circuits is to use phase controlled rectification which can be accomplished by use of, for example, a ring modulator. In operation, the ring modulator has applied thereto a voltage which is to be converted from a.c. to d.c. In addition, a control voltage of the same frequency either in phase or out of phase is also applied to the ring modulator. The ring modulator operates like a controlled reversing switch so that reversal of the applied voltage during alternate half waves of the control voltage occurs. While this approach has a faster response time than the bridge circuit approach for rectifing an a.c. signal, nevertheless the output has a residual ripple. As with the bridge rectifier, output filtering is generally utilized to eliminate the ripple, however, this again inserts a large time constant into the circuit so that it is no longer capable of responding quickly to changes in the incoming a.c. signal. This aspect of operation for both the bridge circuit and for the ring modulator is disadvantageous in the operation of certain devices.
One particular application in which the above described problems is very disadvantageous is in the measurement of temperature of an object, such as a graphite tube in a flameless atomic absorbtion spectrometer, where the temperature is to be controlled automatically by pirometric means. In such devices, a photoelectric detector may be utilized to provide a substantially sinusoidal a.c. signal which indicates the temperature of the graphite tube. it is desirable to utilize this signal to control the apparatus which heats the tube. Accordingly, any time constant between the a.c. signal and the temperature control signal directly affects the temperature control loop. Accordingly, the prior art rectifiers produce unsatisfactory results.
In view of the foregoing difficulties, it is a primary objective of the invention to provide an a.c. to d.c. converter which produces a very smooth d.c. voltage wherein the circuit itself has a time constant which is small compared to the time constant of prior art a.c. to d.c. converters.
This objective and others for the invention are achieved by the circuit according to the invention wherein the incoming a.c. voltage, which is to be converted to d.c., is applied to two different channels, the first channel being associated with one-half wave of the incoming a.c. signal and the second channel being associated with the other half-wave. Each channel includes a resetable integrator which is coupled to the incoming a.c. signal wherein each integrator is operative during a unique half-wave of the incoming signal. The output of each integrator is coupled to a sampling circuit which is operative during the half cycle when the integrator is not operating. Once the integrator circuit output has been sampled, the integrator is subsequently reset so it will again integrate the incoming a.c. signal during the following half-wave.
The output of each sampling circuit is applied to a differential amplifier so as to produce a difference between the two voltages in the sampling circuits. Since the d.c. component in the input signal appears in the same manner in both sampling circuits, when the difference is formed, the d.c. component is eliminated. The residual ripple in the output results from steps in the a.c. amplitude occuring from one half-wave to the next.
The circuit of the present invention is constructed, according to one embodiment, so that it comprises a first and a second channel. A squarewave voltage in phase with the incoming a.c. voltage is supplied inverted to the first channel and noninverted to the second channel. Each channel of the circuit comprises a first monostable multivibrator, the change-state interval of each being equal to the duration of a half-wave of the incoming a.c. signal. Each channel additionally includes a second monostable multivibrator arranged to be triggered by the leading edge of the output signal from the first monostable multivibrator and having a smaller change-state interval. Additionally, each channel includes a third monostable multivibrator arranged to be triggered by the trailing edge of the output signal from the second monostable multivibrator. The change-state interval of the third monostable multivibrator together with the change-state interval of the second monostable multivibrator is selected so as to have a duration not exceeding a half-wave of the incoming a.c. signal. Each channel additionally includes a first controlled switch which is arranged to be closed by the output of the first monostable multivibrator in that channel so as to apply the incoming a.c. signal to an integrator. In addition, each channel includes a second controlled switch controlled by the output of the second monostable multivibrator in the channel wherein the second controlled switch is operative to gate the output of the channel integrator into a sampling circuit at a time when the integrator is not actively integrating. Each channel additionally includes a third monostable multivibrator circuit which is triggered by the output of the second monostable multivibrator circuit and is operative to control a third switch which resets the integrator. The third switch is operative at a time when the second switch is not operative and the integrator is not actively integrating.